1. Field of the Invention
The present invention relates to an image forming device and particularly to an image forming device which can alter the magnification of an image in the main scanning line by making the frequency of an image clock variable.
2. Description of the Related Art
Conventionally, image forming devices are constructed so as to include a photo-scanning device and the like for scanning a light beam emitted from a light source onto a photoreceptor. Both overfield types and underfield types of photo-scanning device exist. In the overfield type of photo-scanning device, as is shown in FIG. 8, a laser beam emitted from a laser diode 100 serving as a light source is changed into parallel light by a collimator lens 102 and then reshaped by a slit 104. It is then guided to a polygon mirror 116 after being transmitted through an expander lens 106, a reflecting mirror 108, a cylinder lens 110, a reflecting mirror 112, and an f.theta. lens 114 comprising a first lens 114A and a second lens 114B.
The polygon mirror 116 is a regular prism, having a plurality of reflective surfaces 116A on the side surface thereof, which rotates at high speed in the direction of the arrow A around the axis of rotation 118A. Accordingly, the angle of incidence of the light beam on each of the reflective surfaces 116A is continually changing and being deflected. The beam width in the scanning direction of the light beam incident on the polygon mirror 116, with which the overfield type of photo-scanning device is equipped, is substantially wider than the width of the reflecting surfaces 116A of the polygon mirror 116. Therefore, the polygon mirror 116 scans the incident light beam so as to cut it, converts the scanning speeds by the f.theta. lens 114 so as to become uniform, and forms an image on the photoreceptor 120 in the main scanning direction. An image is further formed on the photoreceptor 120 in the sub-scanning direction by a cylinder mirror 122 or a cylinder lens.
Moreover, a start of scan sensor 126 and a reflecting mirror 128 are disposed in the vicinity of the start of main scan position of the photoreceptor 120. A start of scan signal is output by a light beam striking the start of scan sensor 126.
States when a light beam 124 is incident on the polygon mirror 116 of the overfield type of photo-scanning device are shown in FIG. 9A and FIG. 9B. FIG. 9A shows the state when the central area of the image to be scanned is being scanned, while FIG. 9B shows the state when edge potions of the area of the image to be scanned or portions outside the area of the image to be scanned are being scanned. Normally, the light beam incident on the polygon mirror 116 is a laser beam oscillating in side single mode and having a Gaussian type of configuration. Because of this, the light beam incident on the polygon mirror 116 is scanned so as to cut out a portion of it and the beam diameter and amount of light vary somewhat because of variations and so on in the width of the incident luminous flux due to the deflections of the polygon mirror 116.
In contrast, as is shown in FIGS. 10A and 10B, in an underfield type of photo-scanning device, when the angle of deflection is large, an eclipse (the area B within the solid line in FIG. 10B) is generated in the light beam incident on the reflecting surfaces 116A of the polygon mirror 116. This causes abrupt variations in the amount of light of the light beam 124.
In contrast to this, in the aforementioned overfield type of photo-scanning device, the amount of light and so on of the light beam 124 does vary somewhat, but not abruptly. This enables the ratio between the scannable width and the width of the image area, in other words, the effective scanning ratio to be made sufficiently large.
However, if the effective scanning ratio is increased, then it may not be possible to secure sufficient time for carrying out processes such as controlling the amount of light from the laser diode 100 (Auto Power Control--APC), which conventionally are executed outside the image area. In particular, in an image forming device which is able to alter the magnification of the image in the main scanning direction by making the frequency of the image clock variable, the effective scanning ratio changes in accordance with frequency of the image clock. It should be noted that, if the magnification of the image in the main scanning direction is reduced, the frequency of the image clock is increased, while if the magnification of the image in the main scanning direction is increased, the frequency of the image clock is reduced.
An explanation will be given below of the changes in the effective scanning ratio in accordance with the frequency of the image clock with reference to the timing charts shown in FIGS. 11A and 11B.
FIG. 11A shows the timing when a start of scan signal (hereinafter abbreviated to SOS signal) output from a start of scan sensor 126 (hereinafter abbreviated to SOS sensor) is output, and the timing when a signal which allows data output in the image area, in other words, an image signal (hereinafter abbreviated to LS (line sync) signal) is output, when the frequency of the image clock is high. FIG. 11B shows the timings of the outputs of the SOS signal and the LS signal when the frequency of the image clock is low.
At this time, the time T to scan the image area of a fixed number of pixels is shortened when the frequency of the image clock is high (T.sub.1 &lt;T.sub.2). The output cycle of the SOS signal is determined by the number of rotations of an unillustrated polygon motor which instructs the rotation of the polygon mirror 116. Normally, the polygon motor rotates at a constant speed so that the output cycle of the SOS signal is constant. Accordingly, the time outside the image area R is short when the frequency of the image clock is low (R.sub.11 +R.sub.12 &gt;R.sub.21 +R.sub.22), and as the frequency of the image clock becomes lower, the time R outside the image area becomes shorter.
FIGS. 12A through 12C show timing charts from the vicinity of the end of the image area until the next SOS signal is output from the SOS sensor 126. FIG. 12A is a timing chart when the image magnification is not altered (nominal state). FIG. 12B is a timing chart when the frequency of the image clock has been lowered in order to increase the image magnification. FIG. 12C is a timing chart when the frequency of the image clock has been raised in order to decrease the image magnification. Note that APC represents a signal instructing control of the amount of light. This signal instructs the execution of auto power control which controls the amount of light from the laser diode 100 serving as the light source. DATA represents a light instructing signal which contains the image data input to the laser diode 100. The portions indicated by the diagonal lines in FIGS. 12A through 12C correspond to the image data input to the laser diode 100.
As is shown in FIG. 12B, when the frequency of the image clock is lowered in order to increase the magnification of the image in the main scanning direction, the ratio of the image area P to the interval between SOS signal outputs, namely the effective scanning ratio, is increased. Therefore, the time R.sub.A from the end of the output of the signal corresponding to the image area P (the image signal) until the next SOS signal is output (namely, the time outside the image area) is shortened (R.sub.A1 &gt;R.sub.A2). Therefore, the time Q.sub.A necessary to execute control of the amount of light from the laser diode 100 cannot be secured, and the control of the amount of light becomes inaccurate. In some cases, when the amount of light from the laser diode is reduced, the SOS signal cannot be output from the SOS sensor 126.
Further, as is shown in FIG. 12C, when the frequency of the image clock is increased in order to lower the magnification of the image in the main scanning direction, the effective scanning ratio is reduced. Therefore, the time R.sub.A from the end of the output of the signal corresponding to the image area P (the image signal) until the next SOS signal is output is lengthened (R.sub.A1 &lt;R.sub.A3). In this case, the times of the forced light for executing control of the amount of light from the laser diode 100 and of the forced light for outputting the SOS signal from the SOS sensor 126 are lengthened, and control of the amount of light is executed in the area corresponding to the image area P when the frequency of the image dock is low (see FIG. 12B). Normally, the inside of a photo-scanning device is light-shielded in order to prevent light from the forced light area which is unrelated to the image from causing turbidity in the image as stray light. However, in an area P', which has the possibility of being an image area, it is not possible to execute light-shielding measures. As a result, unforeseen reflections and the like occur in the photo-scanning device which then generate stray light.